In this paper we present BAT (BMAT Annotation Tool), an open-source, web-based tool for the manual annotation of events in audio recordings developed at BMAT (Barcelona Music and Audio Technologies). The main feature of the tool is that it provides an easy way to annotate the salience of simultaneous sound sources. Additionally, it allows to define multiple ontologies to adapt to multiple tasks and offers the possibility to cross-annotate audio data. Moreover, it is easy to install and deploy on servers. We carry out an evaluation where 3 annotators use BAT to annotate a small dataset composed of broadcast media recordings. The results of the experiments show that BAT offers fast annotation mechanisms and a method to assign salience that produces high agreement among annotators.